A quantitative survey of the biota of intertidal soft substrata on Aldabra Atoll, Indian Ocean

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Abstract
Intertidal, soft substrata on the peripheral reef flat and within the lagoon of Aldabra Atoll were sampled at 24 stations chosen to represent a wide range of habitats. At each station, densities and biomass were estimated from sieving, or from samples of vegetation; by using a random series of quadrats in the case of smaller species; or searches of a wider area in the case of larger, more dispersed species. Where appropriate, the area of substratum covered by plants was recorded and samples of plant material were retained for estimating biomass. The degree of tidal emersion was noted. Estimates of biomass were derived from laboratory determinations of the ash free dry masses of samples of each species, previously fixed in saline formalin and from their estimated densities. Biomass was finally presented as being in units of kcal m-2 from published energy (calorific) values for the different taxa (1 cal = 4.184J). Very approximate estimates of secondary productivity were made by multiplying the total station animal biomass by the P/B ratio of 1.76 derived from Moore (1972). An ordination of the stations produced a graphical display reflecting the different categories of habitat surveyed. The station groupings on the ordination diagram were derived solely from species distributions, thus lending support to our conclusions of how community structure was determined by the environmental conditions. The main categories of habitat were marine angiosperm beds in the lower intertidal, sand flats above the level of angiosperm growth, mobile sand bars and anaerobic mud associated with mangroves. The faunal equitability at each station was studied by using relative abundances of species expressed as their proportional contribution to the total biomass. In all cases, most of the biomass was represented by 3-14 species which tended to have widespread distributions within the area surveyed. Species richness was minimal on the mobile sand bars, increased with the amount of plant cover, and was maximal where angiosperms were covered by a multispecific algal mat. The group of stations with plant cover had a significantly greater animal biomass than the sandy stations. Since this greater biomass was largely distributed among a few dominant species, it was probably due to the increased primary production. In contrast, species richness increased with topographical complexity. The mean estimate of angiosperm biomass (2172 kcal m -2 ) was similar to the value derived from Moore et al. (1968) for a subtidal bed of Thalassia testudinum in Florida. Secondary productivity was least on the mobile sand bars and anaerobic mud ( < 10 kcal cm -2 a -1 ). Sand flats were more productive (10- 100kcal m -2 a -1 ) with values close to those derived from Moore (1972) for subtidal muddy sand in Florida and Massé (1972) for the sublittoral of a clear, sandy bay in the northwest Mediterranean. Secondary productivity was greatest in the angiosperm beds (100-1000 kcal m -2 a -1 ) with values similar to those of intertidal angiosperm beds in Florida (from Moore 1972).

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